Hash Examples - Perl

Hashes are very useful variables and there are many uses for them. Here are a few examples of using hashes to solve common problems.

Creating Readable Variables

The most basic use of a hash is to be able to index into a variable to obtain information using a readable string which is far more user-friendly than using a numeric index as we would with an array. For instance, this program shows that we can create a record of strings representing RGB colors that one might find in an HTML page:

#!/usr/bin/perl -w
# colors.pl

use strict;

my %colors = (
   red    => '#FF0000',
   green  => '#00FF00',
   blue   => '#0000FF',
   white  => '#FFFFFF',
   black  => '#000000',
   purple => '#520063'
);

print "Red is:    $colors{red}\n";
print "Blue is:   $colors{blue}\n";
print "Purple is: $colors{purple}\n";

Notice how the information in the hash is laid out in such a way that it is readable by human beings. It is easy to see that the RGB string for “red” is “#FF0000” and indexing into the hash is the human-friendly$colors{red}.

Executing this code produces the following:

$ perl colors.pl
Red is:    #FF0000
Blue is:   #0000FF
Purple is: #520063
$

“Reversing” Information

Recall the hash we created earlier in this chapter that was a collection of people and where they lived:

%where = (
        Gary     => "Dallas",
        Lucy     => "Exeter",
        Ian      => "Reading",
        Samantha => "Oregon"
);

If you need to turn this hash around to look up people by where they live, you can use a hash in list context that produces a list of key/value pairs, reverse the list with thereverse()function, and then assign it to a new hash.

%who = reverse %where;

Be careful though—if you have two values that are the same, then converting them to keys means that one will be lost. Remember that keys must be unique.

Here is a program illustrating reversing a hash:

#!/usr/bin/perl -w
# reverse.pl

use strict;

my %where = (
    Gary     => "Dallas",
    Lucy     => "Exeter",
    Ian      => "Reading",
    Samantha => "Oregon"
);

my %who = reverse %where;

foreach (keys %who) {
    print "in $_ lives $who{$_}\n";
}

Executing this code produces the following:

$ perl reverse.pl
in Oregon lives Samantha
in Exeter lives Lucy
in Reading lives Ian
in Dallas lives Gary
$

After we assigned to%who, we created a hash indexed by the location producing the name that is the direct opposite of%where, which was indexed by name to produce the location.